Shock absorbing devices are used in a wide variety of vehicle suspension systems for controlling motion of the vehicle and its tires with respect to the ground and for reducing transmission of transient forces from the ground to the vehicle. Shock absorbing struts are a common component in most aircraft landing gear assemblies. The shock struts used in the landing gear of aircraft generally are subject to more demanding performance requirements than many ground vehicle shock absorbers. In particular, shock struts control motion of the landing gear, and absorb and damp loads imposed on the gear during landing, taxiing and takeoff, and during braking.
A shock strut generally accomplishes these functions by compressing a fluid within a sealed chamber formed by hollow telescoping cylinders. The fluid generally includes both a gas and a liquid, such as hydraulic fluid or oil. One type of shock strut generally utilizes an “air-over-oil” arrangement wherein a trapped volume of gas is compressed as the shock strut is axially compressed, and a volume of oil is metered through an orifice. The gas acts as an energy storage device, such as a spring, so that upon termination of a compressing force the shock strut returns to its original length. Shock struts also dissipate energy by passing the oil through the orifice so that as the shock absorber is compressed or extended, its rate of motion is limited by the damping action from the interaction of the orifice and the oil.
While shock struts may effectively absorb energy during aircraft landing events, “hard-landing events” may result in structural damage to the aircraft and/or landing gear assemblies. A landing event is conventionally deemed “hard” (i.e., a hard-landing event) when the pilot declares the landing event to be such. In other words, there is a level of subjectivity in determining whether a landing event is a hard-landing. There are various problems with this method of subjectively. For example, if the pilot declares a hard-landing event, the aircraft must be inspected, tested, and/or analytically reviewed with landing data to verify that the aircraft structures and/or landing gear assemblies were not overloaded. The inspection process and/or analytical review process generally results in the aircraft being out of commission for a period of time and involves the expenditure of time and resources. Often the inspection process results in no structural damage found. Alternatively, if a hard-landing event occurs without the pilot identifying/declaring it as such, structural damage may go unnoticed. For at least these reasons, conventional solutions for detection of hard landings often do not produce reliable results.